Optical proximity correction (OPC) is a photolithography enhancement technique commonly used to compensate for image errors due to diffraction effects or process effects. OPC is needed mainly in the making of semiconductor devices and is due to the limitations of electromagnetic radiation to maintain the edge placement integrity of the original design, after processing, into the etched feature on the wafer. These projected images appear with irregularities such as line widths that are narrower or wider than designed, and are amenable to OPC compensation by changing the pattern on the photomask (or reticle) used for the imaging.
OPC corrects these errors by moving edges or adding extra polygons to the pattern written on the photomask. This may be driven by pre-computed look-up tables based on width and spacing between the features (known as rule-based OPC), or by using compact models to dynamically simulate the final pattern and thereby drive the movement of edges, typically broken into sections, to find the best solution. The objective is to reproduce, as well as possible, the original IC layout drawn by the IC designer into features etched on the die of the wafer.
The workhorse to enable sub-wavelength lithography is referred to as computational lithography (CL). CL makes use of numerical simulations to improve the performance (resolution and contrast) provided by cutting-edge reticles. CL combines techniques including Resolution Enhancement Technology (RET), OPC, and some non-optical portions. Beyond the models used for RET and OPC, CL can include the signature of the scanner to help improve the accuracy of the OPC model, polarization characteristics of the lens pupil, a Jones matrix of the stepper lens, optical parameters of the resist stack, and diffusion through the resist.
For example, in the sub-wavelength lithography era, off-axis illumination such as Annular and Quadropole has been used to print high density features. While the process margin for certain features has been improved, other features notably line-ends and corners, suffer from high sensitivity to focus variation, and thus have a low depth of focus (DOF) which is also known as the depth of field. DOF is the range of distances in object space for which object points are imaged with acceptable sharpness with a fixed position of the image plane. OPC corrects for size only, as it only adjusts the reticle/mask to bring the feature to the designed size at the best focus and dose conditions.